The present invention relates to adsorbents for the non-cryogenic separation of industrial gases and more particularly for the separation of nitrogen by adsorption in gas flows, such as air, and the purification of hydrogen by adsorption of CO and/or N2.
The separation of nitrogen from gas mixtures is the basis for several non-cryogenic industrial processes, among which the production of oxygen from air by a PSA process (Pressure Swing Adsorption: adsorption under modulated pressure) is one of the most important. In this application, air is compressed and conveyed through an adsorbing column having a marked preference for the nitrogen molecule. Oxygen, at approximately 94-95%, and argon are thus produced during the adsorption cycle. After a certain period of time, the column is reduced in pressure and then maintained at the low pressure, during which period the nitrogen is desorbed. Recompression is subsequently provided by means of a portion of the oxygen produced and the cycle continues. The advantage of this process with respect to cryogenic processes is the greater simplicity of the plants and their greater ease of maintenance. The quality of adsorbent used is the key to an efficient and competitive process. The performance of the adsorbent is related to several factors, among which may be mentioned: the nitrogen adsorption capacity, which will be determining in calculating the ideal column sizes, the selectivity between nitrogen and oxygen, which will condition the production yield (ratio between the oxygen produced and oxygen entered), and the adsorption kinetics, which will enable the cycle times to be optimized and the productivity of the plant to be improved.
The use of molecular sieves as selective adsorbents for nitrogen is a well-known technology. The family of zeolites having a pore diameter of at least 0.4 nm (4 xc3x85) has been provided by McRobbie in U.S. Pat. No. 3,140,931 for the separation of oxygen/nitrogen mixtures. The comparative performance of the various ionic forms of zeolites was examined by McKee in U.S. Pat. No. 3,140,933, in particular that of the lithium form presented as the most efficient in terms of selectivity. The advantage of this zeolite has remained limited due to the difficulty in exchanging the faujasite structure into a lithium form. It is known from Chao (U.S. Pat. No. 4,859,217) that the potentialities of such an adsorbent are fully displayed at high degrees of exchange, typically greater than 88%.
Exchange by means of the calcium ion being easier, efforts have been directed towards calcium-exchanged faujasite structures or towards faujasite structures exchanged by means of two divalent ions, calcium plus strontium (see, for example, Patents U.S. Pat. No. 4,544,378 from Coe and U.S. Pat. No. 4,455,736 from Sircar). In the disclosure by Coe, it is indicated that the state of hydroxylation of the exchanged ions is particularly important with respect to the performances and that this state can be obtained by a specific thermal activation.
The purification of hydrogen by adsorption is also an industrial process of great importance. It relates to the recovery of hydrogen from a mixture of several constituents originating from the catalytic reforming of natural gas, plants for the production of ammonia or ethylene units. The principle of pressure swing adsorption (PSA) is applied in order to obtain hydrogen of high purity. The impurities contained in hydrogen are generally composed of CO2, NH3, N2, CO, CH4 and C1-C4 hydrocarbons, at contents ranging from a few ppm to a few percent. In practice, use is made of a bed composed of alumina or of silica gel, for retaining water, of active charcoal, for retaining CO2 and CH4, and of molecular sieve, for trapping CO and N2.
The first industrial plant, which dates from 1967, is disclosed by UCC in U.S. Pat. No. 3,430,418 and, until now, the zeolitic adsorbent used is a molecular sieve of 5A type.
L""Air Liquide has disclosed, in WO 97/45363, a process for the separation of hydrogen contained in a gas mixture contaminated by CO and containing at least one other impurity chosen from the group consisting of CO2 and saturated or unsaturated, linear, branched or cyclic, C1-C8 hydrocarbons, as well as nitrogen, which comprises bringing the gas mixture to be purified into contact with the bed of a first adsorbent selective for at least carbon dioxide and C1-C8 hydrocarbons, then with the bed of an adsorbent specific to nitrogen (capable of adsorbing most of the nitrogen present in the gas mixture), such as zeolite 5A, and, finally, the bed of a third adsorbent which is a zeolite of the faujasite type exchanged to at least 80% with lithium and in which the Si/Al ratio is less than 1.5, in order to remove the carbon monoxide.
In the light of the importance of non-cryogenic processes for the separation of industrial gases employing molecular sieves, the discovery of increasingly high performance adsorbents is an important objective, both for companies which produce gases and for companies which supply molecular sieves.
The present invention deals with agglomerated adsorbents. Conventionally, agglomerated adsorbents are composed of a zeolite powder, which constitutes the active component, and of a binder intended to ensure the cohesion of the crystals in the form of grains. This binder has no adsorbing property, its function being to give the grain sufficient mechanical strength for it to withstand the vibrations and movements to which it is subjected during pressurization and pressure-reduction operations of the column.
Various means have been provided for overcoming this disadvantage of the binder being inert with respect to adsorbing performances, including the conversion of the binder, in all or part, into zeolite. This operation is easily carried out when use is made of clays from the kaolinite family, calcined beforehand at temperatures of between 500xc2x0 C. and 700xc2x0 C. An alternative form consists in manufacturing pure kaolin grains and in converting them to zeolite: its principle is explained in xe2x80x9cZeolite Molecular Sievesxe2x80x9d by D. W. Breck, John Wiley and Sons, New York. The technology in question has been applied with success to the synthesis of grains of zeolite A or X, composed up to 95% by weight of the zeolite itself and of an unconverted residual binder (see, to this end, Howell, U.S. Pat. No. 3,119,660). Other binders belonging to the kaolinite family, such as halloysite, have been converted into zeolite, the addition of a silica source being recommended when it is desired to obtain a zeolite X (xe2x80x9cZeolite Molecular Sievesxe2x80x9d, Breck, p. 320).
Kuznicki and coworkers have shown (U.S. Pat. No. 4,603,040) that it is possible to convert a kaolin agglomerate into zeolite X with an Si/Al ratio equal to 1. The reaction, in order to be virtually complete, that is to say in order to result in the formation of a grain composed of approximately 95% zeolite X, requires some 10 days at 50xc2x0 C., however, which makes the operation unfeasible industrially. It is carried out by combining a maturing period of 5 days at 40xc2x0 C. with a consecutive crystallization at a higher temperature.
JP-05163015 (Tosoh Corp.) teaches that grains of zeolite X with a low Si/Al ratio, of the order of 1.0, can be formed by mixing a zeolite X powder, with an Si/Al ratio of 1, with kaolin, potassium hydroxide, sodium hydroxide and carboxymethylcellulose. Shaping is carried out by extrusion. The grains thus obtained are dried, calcined at 600xc2x0 C. for 2 hours and then immersed in a sodium hydroxide and potassium hydroxide solution at 40xc2x0 C. for 2 days.
By following the teachings of these two documents, it is possible to prepare mechanically strong solids mainly composed of zeolite X, the Si/Al ratio of which is substantially less than that of zeolites X conventionally manufactured by the gel route, the Si/Al ratio of which is between 1.1 and 1.5. These processes are inelegant and suffer either from an excessive reaction time or from the number of stages involved. Moreover, it is to be feared that the heat treatment as claimed in JP 05-163015, after the shaping stage, does contribute to the amorphization of the grain and that the object of the caustic digestion which follows is to recrystallize it, which would explain the slowness of the process.
In the present application, the designation LSX (Low Silica X) is reserved for zeolites X with a low Si/Al ratio, namely zeolites X with an Si/Al ratio of 1, reasonable experimental deviations around this unit value being accepted, lower values very definitely corresponding to inaccuracies in the measurement and higher values corresponding to the presence of inevitable impurities with a higher silica content, and containing sodium ions and possibly potassium ions. It is shown here that it is possible to prepare agglomerated zeolitic bodies composed of at least 95% of zeolite LSX by using a much simpler and faster process and that, from these bodies and by lithium exchange, it is possible to prepare adsorbents which have a particularly outstanding performance not only in nitrogen/oxygen separation but also in nitrogen-carbon monoxide/hydrogen separation.
The process whereby agglomerated bodies made of lithium-exchanged zeolite LSX (hereinafter LiLSX) according to the invention are produced comprises the following operations:
a) bodies made of zeolite LSX are subjected to one or more successive exchanges with a lithium chloride solution at a temperature of approximately 100xc2x0 C.,
and optionally to exchange of the exchangeable cationic sites of the LSX with ions from Groups IA, IIA, IIIA and IIIB of the Periodic Classification, trivalent ions from the series of lanthanides or rare earth metals, the zinc(II) ion, the cupric(II) ion, the chromic(III) ion, the ferric(III) ion, the ammonium ion and/or the hydronium ion, the preferred ions being the calcium, strontium, zinc and rare earth metal ions,
b) the bodies exchanged in a) are repeatedly washed until a low content of chlorides with respect to the solid is achieved (less than 0.02% by weight),
c) the products washed in b) are dried and thermally activated according to a method which does not produce hydrothermal degradation of the zeolitic structure,
the bodies containing zeolite LSX being the products resulting from the following operations:
i) agglomerating a zeolite LSX powder with a binder containing at least 80% of a clay which can be converted to zeolite,
ii) shaping the mixture obtained in i),
iii) drying it and then calcining it at a temperature of 500 to 700xc2x0 C., preferably of 500 to 600xc2x0 C.,
iv) bringing the solid product resulting from iii) into contact with a caustic aqueous solution,
v) washing, drying and activating at a temperature of 300 to 600xc2x0 C., preferably of 500 to 600xc2x0 C.
Conversion of the binder to zeolite takes place during the stage iv) by the action of the caustic solution, which must be at least 0.5 molar and which can be a sodium hydroxide and potassium hydroxide solution in which the potassium hydroxide is present at a maximum content of 30 molar % (with respect to the combined sodium hydroxide+potassium hydroxide). It can be advantageous to use a sodium hydroxide solution.
When conversion to zeolite is carried out with sodium hydroxide, it is particularly advantageous to carry this out on a column, because it is thus possible to remove the potassium from the structure, the advantage being that, during the subsequent lithium exchange, potassium will not be found in the lithium effluents, which accordingly places less of a burden on their selective recrystallization treatment.
In this case, conversion to zeolite is carried out at a temperature sufficient to obtain a reasonable rate of conversion to zeolite.
The clay which can be converted to zeolite belongs to the kaolinite, halloysite, nacrite or dickite family. Kaolin is very simply used.
The lithium exchange operation and any exchange operations for cationic sites detailed above are carried out under conditions well known to a person skilled in the art. They are advantageously carried out on a column, in order to minimize consumption of lithium and any other cations.
The activation of LiLSXs in c) is recommended according to the method which is particularly respectful of the structure, which is activation by hot air in a column according to Patent EP 0,421,875.
The agglomerated zeolitic bodies according to the invention for which exchange with lithium and optionally exchange with one or more ions from Groups IA, IIA, IIIA and IIIB of the Periodic Classification, trivalent ions from the series of lanthanides or rare earth metals, the zinc(II) ion, the cupric(II) ion, the chromic(III) ion, the ferric(III) ion, the ammonium ion and/or the hydronium ion has been carried out, the degree of exchange of which, corresponding to the sum of the cationic sites exchanged in the stage a), is greater than or equal to the equivalent of 80% and preferably greater than or equal to 95% of all the cationic sites of the zeolites,
the lithium representing at least the equivalent of 50% of this total degree of exchange,
it being possible for the calcium to represent at most the equivalent of 40% of this total degree of exchange,
it being possible for the strontium to represent at most the equivalent of 40% of this total degree of exchange,
it being possible for the zinc to represent at most the equivalent of 40% of this total degree of exchange,
it being possible for the rare earth metals to represent at most the equivalent of 50% of this total degree of exchange,
are excellent adsorbents of nitrogen for the separation of gases from air and excellent adsorbents of nitrogen and/or of carbon monoxide for the purification of hydrogen; agglomerated zeolitic bodies for which the degree of exchange, corresponding to the sum of the cationic sites exchanged in the stage a), is greater than or equal to the equivalent of 95% of all the cationic sites of the zeolites exhibit a nitrogen capacity at 1 bar and at 25xc2x0 C. greater than or equal to 26 cm3/g and they are particularly preferred by the Applicant. The adsorption processes employed are generally of PSA or VSA type.